NATUKAL RHYTHM. 171 



specimens of Aurelia into a solid block of ice. Of 

 course, as sea-water had to be employed, the cold 

 required was very considerable ; but I succeeded in 

 turning out the Medusae encased on all sides in a 

 continuous block of sea- water. By now immersing 

 this block in w^arm water, I was able to release the 

 contained specimens, which then presented a very 

 extraordinary appearance. The thick and massive 

 gelatinous bell of a Medusa is, as every one knows, 

 chiefly composed of sea-water, which everywhere 

 enters very intimately into the structure of the 

 tissue. Now, all this sea-water was, of course, 

 frozen in situ, so that the animals were every- 

 where and in all directions pierced through by an 

 innumerable multitude of ice-crystals, which formed 

 a very beautiful meshwork, pervading the whole 

 substance of their transparent tissues. 



These experiments were made in order to ascertain 

 whether the Medusae, after having been thus com- 

 pletely frozen, would survive on being again thawed 

 out, and, if so, whether the freezing process would 

 exert any permanent influence on the rate of their 

 rhythm. Now in all the cases the Medusae, after 

 having been thawed out, presented a ragged ap- 

 pearance, which was due to the disintegrating 

 eflfect exerted by the ice-crystals while forming in 

 the tis^es ; yet notwithstanding this mechanical 

 injury superimposed on the physiological effects of 

 such extreme cold, all the Medusre recovered on 

 being restored to sea-water of the normal tem- 

 perature. The time occupied by the process of 

 recover}^ varied in different individuals from a few 



